Intrusion detection devices are used in a large variety of industrial and security applications. Many different techniques are used of which most are based on specific applications of microwave, radar, optical, acoustic, sonar, or ultrasonic technologies. Typical industrial applications include automatic door openers, detection of goods on an assembly conveyor, and devices that turn on lights upon entering a room. Typical security applications include sensors to detect the presence of unauthorized individuals or vehicles in a specific area.
Many intrusion detection applications can be solved using simple devices and methods such as interrupting a light beam or incorporating motion detectors based on optical or microwave techniques. However, there are other applications in which it is desired to track the intruder or identify the type of intruder once it enters the surveillance area. It is these types of applications that are targeted by this invention.
Tracking the location of an intruder is much more difficult than simply detecting its presence. Whereas simply flooding an area with a microwave, optical, or ultrasonic beam and detecting the doppler frequency shift due to intruder movement works good for detecting the presence of an intruder, this simple technique cannot be used to pinpoint its location or identify the type of intruder.
To track an intruder, a video camera or some type of scanning system such as radar is typically required. Cameras cannot be used when the optical clarity of the area is poor such as in fog or in underwater applications. In scanning systems, large mechanical scanning antennas or phased arrays are typically required to obtain the narrow beam widths needed for high directional sensitivity.
Although the present invention could be applied to many different intrusion applications, the method and apparatus to be described in this disclosure was motivated by the desire to detect the intrusion of sharks into a swimming area such as a public beach.
The incidence of shark attacks on swimmers has increased over the years as more and more people seek recreation at beaches around the world. Although attacks are still rare, they almost always cause serious injury or death. Therefore, it would be beneficial to municipalities, hotels, and resorts to have the capability of detecting the presence of sharks and alerting swimmers whenever they entered the swimming area.
For this intrusion application, both the location and identification of the intruder are required. Since large fish, dolphins, sea lions, and the like may also be present in waters frequented by sharks, a means of discriminating between these different types of intruders is also required. Optical (camera) techniques cannot be used because of poor water clarity at most beaches and low ambient lighting at all beaches at night. Large scanning or phased array sonar systems could be used but their cost would most likely be prohibitive.
Altes in U.S. Pat. No. 3,952,280 describes a sonar technique in which an area is illuminated with broad band radiation and the reflected signal is passed through various filters in an attempt to discriminate between various targets. The location of the target is not indicated and it is highly unlikely that this system could discriminate between similar size objects such as sharks and dolphins.
Accordingly, it is the object of the present invention to provide a new and improved method, of detecting, tracking, and identifying intruders into an area under surveillance.